1. Field of the Invention
The present invention relates to a charge-coupled device (CCD), and more particularly, to such a device which has improved transfer efficiency.
2. State of the Prior Art
It is known in two-phase CCD devices to employ either a true two-phase structure with two distinct potential levels under a single gate electrode or a pseudo two-phase structure in which there is a uniform, but distinct, potential level under each of two electrically-connected gate electrodes which form one clock phase. In applications involving long device cells and in high-speed applications, the transfer efficiency of these conventional structures is not entirely satisfactory. Thus, ways have been sought to improve the transfer efficiency in CCD's while at the same time not unduly complicating the process of making the device.
U.S. Pat. No. 3,767,983, is directed to improving the transfer efficiency in a charge transfer device of the bucket brigade type. The device disclosed in this patent includes two different threshold voltages in the transfer region between each pair of successive storage sites. There is a substantially abrupt transition between the two different threshold voltages in the transfer region. The abrupt transition is provided to improve the transfer efficiency of the device by solving the problem of feedback voltage which occurs between the transferor zone (source) and the transferee zone (drain). Such a problem does not exist in CCD's, however, and thus, this patent does not provide a solution to the problem of increasing the transfer efficiency in a CCD.